1. Field of the Invention
This invention relates to apparatus, which can rapidly and precisely position a lens or other optical element along an axis.
2. Description of Related Art
Lasers are used in a variety of fields, from surveying, to supermarket bar-code scanners, to optical disk drives such as CD and DVD drives. One particular class of laser applications involves scanning the laser beam using X-Y galvanometer scanners, for the purpose of marking or cutting material, or for the purpose of creating a visual image.
When lasers are used for marking or cutting, it is typical that the laser beam is deflected by X and Y scanners, and then sent through a “scan lens,” which is usually implemented as an F-Theta lens or as a Telecentric lens. The lens is used to focus the laser beam onto the material being marked or cut. Normally the beam diameter exiting from the laser is between 6 and 12 mm, and it is necessary to focus the beam onto the material to achieve a high enough energy density in order to mark or cut the material.
The “scan lens,” such as an F-Theta or Telecentric lens, is a significant part of the cost of the overall laser marking system. The lens generally must be two to four inches in diameter, and is often made of exotic materials in order to pass the wavelength of interest. Moreover, the F-Theta and Telecentric lenses create a focus that is onto a planar target. So this means that the material being marked or cut must be flat, so that the beam will remain in focus all along the material's surface. It is not possible to mark onto a non-uniform surface such as a cylindrical soda can or wavy product. This planar and non-changeable focus distance along with the high cost of a scan lens are two disadvantages of using scan lenses. It is therefore desirable to have a system that could provide dynamic focus while the beam is being scanned so that non-uniform surfaces can be marked.
Another application for lasers is for display applications. Laser displays are used for many things, including optical layout templates. In a related application a laser display can be used for entertainment applications, for example, to project company logos, animated cartoon figures and the like, and also projected directly into an audience. When lasers are projected into an audience, referred to as “audience scanning.”
When creating a displayed image of a company logo or cartoon, typically the “raw beam” is used out of the laser, then sent to a X-Y scanners. Vector graphics being sent to the X-Y scanners from a computer then creates the image on the target surface. Focusing, defocusing, or changing the beam diameter during the X-Y scanning is not typically done in laser display projectors known in the art. As a result, the image has a roughly constant size laser beam across the entire projection surface. However, it is desirable to have a device that provides variable focus or defocus capability, such that certain parts of the projected image can have a larger spot size (for example, big blushy cheeks on a woman's face) while other parts of the image can have a very small spot size (for example, eye lashes on a woman's face).
Likewise, when creating an audience scanning display, normally the “raw beam” is used right out of the laser, sent to an X-Y scanner, and then directly into the audience. In audience scanning laser projectors known in the art, focusing, defocusing, or changing the beam diameter of the X-Y scanning beam is typically not done. Therefore, just as in the case of a typical laser graphics projector discussed above, the entire audience receives the same diameter laser beam at all times and all places in the projected display. However, it is desirable to have a device that can provide variable focus or defocus such that parts of the image can have a higher beam diameter, and other parts could have a lower beam diameter. With audience scanning applications this can be especially important, because the safety of the laser beam is increased as the diameter of the laser beam is increased within the audience. If a variable focus device were used, it could increase the beam diameter for areas of the laser projection where audience members are particularly close to the laser projector, and thus safety could also be increased.
Several devices are known that try to create a precision, variable-focus system for a laser beam. These devices have generally taken one of two forms. One form is where a normal galvanometer scanner (which is a rotary device) is employed into a system that uses a rotary-to-linear mechanical translator, such as a taut-band Rolamite. The motion of the moving member is then restricted such that it can only move axially, and not radially or rotationally, by a rod-bearing system. A lens or other optical element is then mounted to the moving member. In this way, an off-the-shelf galvanometer scanner can actually be used to move a lens in a linear fashion, instead of moving a mirror in a rotary fashion, as is typically the case for galvanometer scanners. Although galvanometer scanners are off-the-shelf devices, they really were not designed to be applied as lens translators. As a result, there are a number of problems with this technique. The rod bearings eventually wear out, and also have limited maximum speeds. And the linkage between the rotary scanner shaft and linear sliding member cannot be made infinitely stiff; so resonance problems will prevent the speed of such a device from being very high.
Another approach for creating a precision, variable-focus system for a laser beam is to use a moving-coil actuator coupled to a rod-bearing system similar to that described above. The rod-bearing system allows the coil and moving optical element to move axially, but neither radially nor rotationally. Oftentimes, the lens is located in the center of the moving coil. Performance of this type of system is better than the approach described above, but still not satisfactory for some applications, including laser display and audience scanning applications. In one particular prior-art system, wherein the moving element and coil ride along a rod-bearing system, the maximum slew rate achievable is 1600 millimeters per second, and maximum acceleration is 50 Gs.
Certainly the use of a rod-bearing system is a great disadvantage for a Z-axis focusing system for certain applications. As a result, there have been attempts to replace the linear bearing system with flexures of various forms, such as a flat-spring flexure or even wires used as a flexure. However, known flexure systems exhibit self-resonances that prevent the overall Z-axis focusing device from achieving speeds that are anywhere near the frequency of the flexure self-resonances.
In one configuration using metal flexures, an undesirable additional motion is imparted to the moving member. For example, one such approach uses three flat-spring flexures arranged in a triangular fashion. As the moving element is moved along the Z-axis, the flexures maintain axial motion while restricting radial motion. However, due to the triangular and flat-spring nature, as the element is moved, a parasitic rotational motion is also imparted onto the member as it is moved axially. The net result appears as a “screwing” action, which is undesirable when compared with pure linear motion.
In another configuration commonly employed in CD and DVD drives, simple wires are used to restrict the motion of the moving element. However, the diameter of the wires must be quite small in order to allow axial motion, and thus the self-resonant frequency and stiffness in the radial direction are not sufficient for laser display or audience scanning applications.
Whether implemented as a rotary-to-linear device or a moving coil device, there is one thing that currently known systems have in common, and that is that the moving member itself has a lot of mass. For example, within industrial Z-axis focusing devices used for laser marking and cutting, the lowest typical moving mass you can find is at least 20 grams, and a moving mass of 50 grams is much more common. Such a high moving mass is detrimental to achieving very high speeds. Even the in Z-axis focusing devices used in CD and DVD players, the moving mass is typically around 0.3 grams, which is a lot of mass when compared with the force that CD/DVD actuators produce (typically less than 0.2 newtons). Thus the frequency attainable by Z-axis focusing devices at present is insufficient for use within those applications that require very fast dynamic focus action, such as laser displays and audience scanning.
Also, in known devices that use a single magnet concentric with the optical axis, the magnet is positioned inside the coil assembly, with an outer steel “back iron” on the outside of the coil assembly. Since the magnet must have a bore therethrough for passing light, there really is insufficient magnet material to generate substantial flux. As a result, the flux density in the magnetic circuit is sub-optimal; so force is also sub-optimal. Any attempt to increase the outside diameter of the magnet and thus increase flux density would come at the cost of also increasing the coil outside diameter, which increases the moving mass and is thus undesirable.